Corrosion testing apparatus

ABSTRACT

A rotational test electrode assembly for use in a corrosive fluid environment as shown in  FIG. 1  that includes a generally cylindrical heat and electrically conductive member ( 110 ) having an annular portion ( 112 ) and a solid portion ( 114 ): a heating device ( 130 ) positioned inside of the annular portion and in heat exchanging relation with the solid portion of the conductive member; a corrosion resistant external protective member ( 140 ) that surrounds a portion of the heat conductive member in close-fitting relationship; mounting means for attaching a rotational electrode ( 150 ) in close fitting heat and electrically conductive relation, the electrode being selected from the group consisting of cylindrical and disk electrodes; and an electrical connector for receiving a plurality of external electrical connectors that is mounted on the protective member opposite the portion of the conductive member on which the rotational electrode is mounted.

FIELD OF THE INVENTION

This invention relates to apparatus useful in investigating corrosionphenomenon that effect industrial equipment and, more particularly, toapparatus that is utilized in determining the electrochemical behaviorof metals and the performance of corrosion inhibitors under controlledconditions.

BACKGROUND OF THE INVENTION

A wide variety of apparatus and methods have been developed by corrosionand design engineers for evaluating the effect of corrosion-producingfluids passing in contact with industrial equipment, such as heatexchangers, reactors, pipelines, and the like. Apparatus and methods forin situ testing of effects on pipelines are known, as are numerouslaboratory techniques. Laboratory bench scale test devices known to theprior art include rotating disk and rotating cylinder specimens that areimmersed in corrosive fluid media to determine the effects on, e.g., thetypes of metals that come into contact with comparable fluids underindustrial conditions. The rotational movement of the specimens can bevaried to simulate actual hydrodynamic conditions.

Many production processes and material transport systems in industrialplants involve heat transfer across a metal-fluid interface and masstransfer to or from that interface, including the buildup of scaledeposits and loss of material due to corrosion. Materials selected forindustrial applications must, therefore, be able to withstand or atleast resist adverse effects that are initiated or accelerated by heatand mass transfer. In order to make the optimum choices, corrosion anddesign engineers need an understanding based on data relating to theeffects of heat and mass transfer on material degradation fromcorrosion. Little investigative work on corrosion and corrosionprevention under heat and mass transfer conditions has apparently beenreported in the literature.

It is therefore an object of the present invention to provide corrosiontesting apparatus to examine the electrochemical behavior of metals andthe performance of organic and inorganic inhibitors or passivators underconditions of controlled and quantified heat and mass transfer.

Another object of the invention is to provide a means for obtaining datato quantify the effect of various conditions and factors by simulatingindustrial conditions effecting the corrosion behavior of metals and theperformance of chemical additive corrosion inhibitors.

A further object of the invention is to provide an apparatus thatpermits the bench scale investigation of industrial corrosion factorswith minimum costs and that permits the testing and evaluation ofinhibitors without risk of direct or indirect damage to the industrialfacilities.

SUMMARY OF THE INVENTION

The above objects and other advantages are achieved by providing asingle rotating electrode apparatus that is configured to receive eitherof a pair of rotating electrodes in heat conductive mounting relation inorder to obtain quantified data under heat transfer conditions. Thefirst of the pair of electrodes is a rotating disk electrode (RDE) andthe second is a rotating cylinder electrode (RCE). Both the RDE and RCEare interchangeably mountable for rotation on the same rotationalelectrode supporting shaft in a test stand to provide economy ofmaterials and measuring devices.

In a particularly preferred embodiment, the interchangeable rotatingelectrode supporting shaft is provided with an internally mountedheating element in the form of an electrical resistance heating device.The heat conductive member can be fabricated from brass in the form of ahollow cylinder proportioned to receive the heating device and itsassociated electrical leads. The heating device is positioned proximateto a metal heat conductive supporting member that efficiently conductsthe heat generated to an external surface that is in contact with eithera disk electrode or a cylindrical electrode specimen.

In a particularly preferred embodiment, a plurality of thermocouples orthermistors are embedded in the conductive member to provide temperaturereadings at positions closely adjacent to the point of attachment of therotating electrode, as well as proximate and displaced from the heatingdevice. The leads from the thermocouples also extend axially to theupper end of the rotating electrode shaft to a plug, socket or otherterminal connection. The thermocouples are connected to a remotetemperature display and recording device to provide the necessary datafor controlling the power to the heating device to meet the desiredtemperature of the specimen electrode.

An electrically conductive lead is also attached to the metal conductivemember on which the electrodes are mounted in heat and electricalconductive relation. This lead also extends to the plug or sockettermination for subsequent connection to a power source.

In order to meet the electrical power requirements of the rotatingelectrode assembly during operation, the drive shaft that rotates theassembly is provided with a slip-ring assembly having a plurality ofelectrical conductors corresponding to the conductors required toprovide current to the electrode support, heating device, and for eachof the plurality of thermocouples mounted in the conductive member. Eachof the leads from the slip-rings is terminated in a plug or socket formating engagement for its counterpart in the end of the rotatingelectrode shaft. As an alternative, the plurality of leads from theslip-rings and the rotating electrode can be individually joined byappropriate insulated connectors.

A corresponding brush set is provided with appropriate leads to providethe necessary electrical power input to the slip-rings during rotationof the driveshaft. It will also be understood that the leads from therespective brushes are operably connected to one or more units fordisplay and, optionally, recording of the temperature of each of theplurality of thermocouples; a separate power control and display unit isconnected to the leads of the heating device. A separate power controlunit and display is also provided for the working electrode. A singleground connection is utilized in a preferred embodiment in order tominimize the number of wires required.

In order to protect and isolate the heat conductive member from thecorrosive fluid in which the unit is immersed, it is provided with afluid-tight protective cover or housing. The protective cover materialis also to be electrically non-conductive in order to isolate therotating electrodes from stray currents.

The protective layer can be selected from such highly corrosionresistant and electrically insulative polymeric materials aspolytetrafluoroethylene (PTFE). The protective housing is preferablyprovided in the form of a hollow cylindrical member with a wallthickness that provides a rigid construction. The cylindrical metalconductive member can then be positioned inside of the close-fittinghollow polymeric cylinder.

In order to provide a rigid point of attachment for the driveshaftcoupling, the hollow protective housing is preferably extended wellabove the end of the internal conductive member to form a portion thatalso extends well beyond the top of the polarization cell when therotating electrode is in an operational position. In order to provideadditional rigidity to the upper end of the protective housing member atthe point of attachment to the driveshaft coupling, a close-fittingcylindrical metal sleeve member having a flanged top is inserted intothe hollow end of the protective member. This internal sleeve isdesigned to provide sufficient strength and rigidity to permit setscrews or other attachment means to rigidly secure the driveshaft to therotating electrode shaft. In order to maintain electrical isolation ofthe electrode, the reinforcing sleeve should not come into contact withthe heat conductive member.

The lower portion of the metal conductive member is threaded externallyin order to receive a cooperatively threaded rotating cylindricalelectrode element that serves as the test specimen. In the preferredembodiment, the rotating cylinder electrode is threaded onto theconductive member to a position that substantially surrounds theinternal heating device to minimize the distance heat must betransmitted through the conductive member to elevate the temperature ofthe rotating cylinder electrode specimen to the desired degree. In thisregard, thermocouples or thermistors are positioned in the conductivemember adjacent the mid-point of the rotating cylinder electrode when itis in position for operation.

In order to protect the lower portion of the cylindrical heat conductingmember, an internally threaded polymeric protective cap in the form of acup is positioned to provide a fluid-tight seal to the base of therotating cylinder specimen when assembled to the conductive member. Itwill also be understood that the upper portion of the protective memberforms a fluid-tight seal with the upper rim of the rotating cylinderwhen it is threaded onto the conductive member. Internally threaded capsof different depths can be provided to accommodate rotating cylinderspecimens of different axial lengths. The direction of the respectivethreads is opposite that of the direction of rotation to insure that theelectrode will not become loosened during operation.

In order to securely mount the rotating disk electrode, an internallythreaded opening is provided in a smooth flat surface at the lower endof the supporting shaft to receive a cooperatively threaded attachmentshaft projecting from the upper surface of the RDE. When the RDE istightly threaded onto the end of the heat conductive member, the matingsurfaces provide an efficient heat conducting interface boundary.

In a particularly preferred embodiment, a recessed or setback shoulderis provided at the upper end of the RDE in order to provide a matingsurface for engagement with the projecting end of the hollow cylindricalprotective member. This mating engagement is also intended to provide afluid-tight seal that will prevent corrosive fluid incursions andcontact with the metal conductive member.

The data obtained from the apparatus is applied utilizing knownalgorithms and methodologies known to those of ordinary skill in the artto calculate the surface temperature of a rotating cylinder electrodeand/or disk that is transferring heat to the fluid in the test cell.Further calculations are undertaken to determine the effects of laminarflow conditions and turbulent flow conditions on the vulnerability ofthe test cylinders and disks to the corrosive fluid(s) in the testapparatus. Similarly, the data obtained is used to determine the effectsof corrosion inhibitors and metal passivation additives under varioustemperature and simulated flow conditions.

The apparatus and method of the invention provide for the heating ofeither the RDE or the RCE, or both simultaneously, to a temperature thatexceeds the temperature of the surrounding fluid in the test solutionchamber. The invention thereby allows simulation of conditions of heattransfer such as those that exist in heat exchangers of the tube typewhere the temperature at the inlet end can vary considerably from thatat the discharge end. Furthermore, the temperature differentials betweenthe heat exchanger tubes and the surrounding fluid also changes betweenthe inlet and outlet ends. Prior art tests and/or calculations that arebased upon only a single temperature will only reflect the conditionsexisting at a particular portion of a pipe or tube in the heatexchanger. Utilizing the apparatus and method of the present invention,the temperature of the rotating cylinder electrode can be varied byadjusting the heat generated by the heating device over a range oftemperature differentials, thereby more accurately replicating actualindustrial conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described below and with reference to theattached drawing sheets in which:

FIG. 1 is a schematic elevational view, partly in cross-section, of anembodiment of the invention used with a cylindrical electrode;

FIG. 1A is a further preferred embodiment similar to FIG. 1 illustratingan axially elongated cylindrical electrode for use in the invention;

FIG. 2 is a schematic illustration of the invention similar to FIG. 1for use with a disk electrode;

FIG. 3 is a schematic illustration of an elevational view of therotating electrode test assembly with a cylindrical electrode in placeand operably connected to a conventional test stand for providingelectrical power and rotation to the assembly; and

FIG. 4 is a schematic illustration of a side elevational view of therotational test electrode assembly utilizing a disk electrode inoperating position in a typical test cell.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated the rotating testelectrode assembly of the invention that is arranged for use with acylindrical electrode specimen. Conductive member 110 consists ofupwardly extending annular member 112 defining an opening 116 andterminating in an integral solid base portion 114. The lower portion ofthe conductive member 110 is provided with external threads thatcooperatively engage mating internal threads on cylindrical electrodespecimen 150. A protective member 140 extends from the upper matingsurface of electrode 150 to a terminus above the end of annular portion112.

A heating element 130 is securely positioned in annulas 116. Electricalleads extend from the heating device 130 to a electrical terminal in theform of socket 138 positioned in the upper end of assembly 100.

In a preferred embodiment, a plurality of thermocouples 113 arepositioned in the conductive member 110 proximate the heating device 130and the lower end of solid portion 114 proximate the point of attachedof the disk electrode, as described below in connection with FIG. 2.

With continuing reference to FIG. 1, a second portion of the protectivemember 140 takes the form of an interiorally threaded polymer cup 144which is received on the lower end of conductive member 110 to make afluid-tight contact with the bottom edge of electrode 150.

With further reference to FIG. 1, a reinforcing flanged metal sleeve 160is secured in the upper open end of protective member 140 to provide arigid point of attachment for a coupling member, as will be describedfurther below. Electrical leads extending from socket 138 are connectedto the working electrode at a point 118 in the side wall of annularportion 112, and to the thermocouple leads.

It is also to be noted that an air gap 148 has provided between thereinforcing sleeve 160 and the hollow cylindrical portion 112 in orderto electrically isolate the working electrode 150 from any straycurrents that might be produced by the motor or other electricalcontrols used to power the apparatus.

As will be seen from FIG. 1A, the axial dimension of the cylindricalelectrode can be extended as in 150A, while the protective cup portion144A will be reduced in height. This feature of the invention providesadded flexibility to the testing of cylindrical electrodes, while stillmaintaining the ability to utilize the rotational test electrodemounting shaft with a disk electrode, as will be described below.

Referring now to FIG. 2, it will be seen that the same principalstructural elements are utilized for mounting disk electrode 156 on thelower end of conductive member 110. The threaded orifice 114 receivescorrespondingly threaded shaft 152 extending from the upper surface ofdisk electrode 156. A second portion of the protective member in theform of sleeve 142 is mounted on the lower portion of conductive member110 and engages the shoulder 154 when the unit is assembled foroperation. In this embodiment, it will be understood that thecylindrical electrode 150 has been removed and in its position has beenplaced protective sleeve 142 which forms a fluid-tight seal with thelower portion of the protective member 140.

The use of the rotational test electrode assembly will be described inconduction with FIG. 3 in which a rotating disk is to be utilized. Asshown in FIG. 3, the rotational assembly 20 comprises a variable speedmotor 22 with speed indicator/controller 21. A mounting collar 24couples the output shaft of motor 22 to drive shaft 26, which can be ofany desired length to conveniently position the rotational assembly 20for the manual attachment and removal of the respective rotationalelectrodes 100, and their placement in the polarization cell 40.

With continuing reference to FIG. 3, the drive shaft 26 passes through,and is supported for rotation by bearing member 28 that is mounted onsupporting plate 36 which can be utilized to conveniently mount theapparatus on a bench, rack or other stable mounting device forconvenient access.

A plurality of electrically conductive brushes 30 are mounted inslip-ring mounting member 32 on the drive shaft 26. As shown best in thecross-sectional view of FIG. 3A, the free-ends of the generally U-shapedbrush elements 31 contact the rotating surface of the slip-rings.

The lower end of drive shaft 26 extending from the slip-ring assembly 32is hollow to receive the plurality of electrical leads (not shown) thatare connected to the plug 38 that is fitted to the end of the shaft 26.Plug 38 mates with socket 138 that is fitted to the end of rotatingelectrode shaft 140.

Referring to the schematic illustration of FIG. 4, the corrosion testingapparatus 10 of the invention comprises a rotational electrode assembly100, the lower portion of which has been fitted with a rotatingcylindrical electrode (RCE) 150 in accordance with the descriptionprovided above, particularly with reference to FIG. 1.

The rotational assembly 100 is shown positioned in polarization cell 40.As shown FIG. 4, polarization cell 40 includes a first chamber 42 forreceiving the rotating electrode and a second, smaller chamber 44separated by a sintered glass member 46 located in a conduit joining thetwo chambers. The larger chamber 42 is provided with coil 48 throughwhich can be passed a heat transfer fluid provided from an externalsource (not shown) in order to maintain a predetermined desiredtemperature differential between the heated electrode and the fluid.

A reference electrode 52 is introduced through a fluid-tight fitting 50in a sidewall of chamber 42 or, alternatively, through a similar fitting(not shown) in removable cover 60. A counter electrode (not shown) ispositioned in chamber 44.

Removable cover 60 is received in close-fitting relation over the openend of chamber 42 and is preferably provided with a plurality ofopenings for receiving in fluid-tight relation the shaft of therotational assembly 20, as well as auxiliary devices that can include,e.g., a thermometer or other temperature sensing device; inlet andoutlet tubes 48A and 48B, respectively, of cooling coil 48; gas inletand removal conduits, e.g., to provide a nitrogen atmosphere forexclusion of oxygen, and to remove any gaseous by-products generatedduring operation of the apparatus; and to insert the reference electrodeand/or other electrodes and probes that may be required for datacollection and for alerting the conditions with the vessel 42.

Appropriately configured stoppers and/or seal members 62 are fitted intounused openings or around projecting tubes and the rotational assembly100. A stopper 62 can be inserted to close the open end of smallerchamber 44.

EXAMPLE

A test cell in accordance with the invention consisting of twocompartments was constructed with a first flanged working electrodecompartment having a capacity of 2 liters and a 12 cm. diameter; theadjacent second counter electrode compartment being of the same height,had a 2.5 cm. diameter. The two compartments were connected with a tubeof 25 mm diameter containing a full bore sintered class disk. The testcell was fabricated from a corrosion resistant, electrically insulativepolymeric material.

A rubber stopper 50 was secured in a flanged opening in a side walladjacent the bottom of the first cell. A capillary tube was passedthrough the stopper and connected to the reference electrode 52 with itstip that served as a Luggin probe being centrally positioned in thereaction vessel. The cell is carefully filled with a corrosive fluidsample that has been obtained from a heat exchanger feedline.

As in the embodiment illustrated in FIG. 4, a Luggin probe is shownpositioned in close proximity to the RCE specimen. By way of background,the Luggin probe consists of a capillary tube which extends into thecell to a position proximate the electrode, the overvoltage of which isto measured. The capillary tube is connected by a salt bridge to areference electrode, for example, to a calomel reference electrode,located outside the electrolytic cell. The Luggin probe is suitable forobtaining intermittent measurements, which is sufficient for laboratorytesting and data collection purposes. When the RDE is utilized theLuggin probe is positioned below and proximate to the underside of thedisk.

The polarization cell cover 60 is positioned below the lower portion ofdrive shaft 26, it being understood that the cover does not rotate andis large enough to receive the rotating shaft and an attached RDE or RCEwhen the polarization cell is brought into operating position below thesupport plate 36.

The test cell compartment 40 was covered with a 140 mm diameter flangedlid 60 having a 40 mm diameter central opening to receive the rotatingassembly, electrode and driveshaft. Two other 20 mm adjacent openingsare provided for use as may be needed for specific tests. The inlet andoutlet tubes for the cooling coil were passed through the side wall ofthe first compartment (not shown).

The rotating electrode supporting shaft 100 is fitted with an internallymounted electric resistance heating device. A protective insulatingcover fabricated from PTFE 140 is employed to isolate the conductivemember from the effects of the corrosive fluid in which the rotatingelectrode is to be immersed.

The rotating electrode assembly 100 is joined to the electrical plug 38through socket 138. The electrode coupling collar 54 that is mounted onthe lower end of drive shaft 26 is secured by thumbscrews 55 to theupper end of shaft 140.

The rotating electrode assembly 100 is passed through central opening 64in cover 60 and sliding seal 62 is put in place. The power to theheating device 130 is turned on and thermocouple readings are observedto determine that the RCE has reached the desired temperature.Thereafter, the power to the drive motor 22 is adjusted by controller 21to obtain the desired revolutions for the testing of the RCE.

After the test has been concluded, the seal 62 is removed and theelectrode assembly 100 is withdrawn from the test cell. The thumbscrews55 on the electrode coupling 54 are released and the socket 138 isseparated from the electrical plug 38. The assembly 100 is then washedand cleaned to remove any residues of corrosive fluid and disassembledto recover the electrode 150 for further testing and analysis.

As will be understood from the above description, the apparatus of theinvention provides the following benefits and advantages for both theRDE and RCE devices in a variety of modes of operation:

-   -   1. a uniform heat flux emanates from the surface of the        cylindrical or disk specimen to the surrounding fluid during        operation in the polarization cell;    -   2. a uniform temperature is provided over the entire surface of        the specimen;    -   3. a uniform boundary layer thickness is presented over the        entire specimen surface;    -   4. the edge of the rotating disk or cylinder shaft has no effect        on the uniformity of the boundary layer conditions;    -   5. no electric current flows from the drive motor to the working        electrode; and    -   6. cylindrical specimens having different heights and associated        surface areas can be used interchangeably in a single rotating        electrode mounting member.

The apparatus of the invention is prepared for operation and recordingof data by filling the test cell with the test fluid, e.g., a corrosiveliquid of known composition at a predetermined temperature. The heattransfer fluid is circulated through coils 48 at the same predeterminedtemperature. The control electrode is inserted into chamber 44 and therotational electrode fitted with a cylindrical electrode specimen asshown in FIG. 3 is passed through the central opening 64 in cover 60 forimmersion in the corrosive test liquid. The Luggin probe is fittedthrough seal 50 and positioned proximate the cylindrical specimen 110.Seal 62 is positioned around the rotating shaft assembly 102. As will beunderstood from the above description, a single rotational shaft isprovided on which can be installed either a disk electrode orcylindrical electrode specimen. The apparatus is capable of evaluatingthe electrochemical behavior of metals and the performance of corrosioninhibitors under static conditions, hydrodynamic conditions of laminarflow and turbulent flow, isothermal and heat transfer conditions, andcombinations thereof.

The apparatus permits the interfacial heat transfer coefficient andinterfacial temperature for the cylindrical electrode specimen to beestimated by conventional mathematical modeling and the utilization ofanalogies among the transport phenomenon for momentum, heat and masstransfer. The apparatus permits data to be collected that is requiredfor conducting quantitative corrosion aid corrosion preventive researchfor industrial application with a minimum of data manipulation.

It will also be understood from the above description that the apparatusand method of the invention is particularly useful for evaluatingcorrosion conditions and the effect of inhibitors as appliedspecifically to industrial cooling systems, e.g., heat exchangers. Theapparatus and method of the invention also provides a reliable,reproducible and inexpensive means for evaluating the inhibitivecharacteristics and compatibility of any of the numerous chemicalcompounds utilized industrially under conditions that simulate closelythose of flow, mass and heat transfer in specific industrialapplications.

As will be apparent from the above description to one of ordinary skillin the art, further modifications can be made to the assembly withoutdeparting from essential features of the invention as defined in thefollowing claims.

1. A rotational test electrode assembly for use in a corrosive fluidenvironment comprising: a) a generally cylindrical heat and electricallyconductive member having an annular portion and a solid portion; b) aheating device positioned inside of the annular portion and in heatexchanging relation with the solid portion of the conductive member; c)a corrosion resistant external protective member that surrounds aportion of the heat conductive member in close-fitting relationship; d)mounting means for attaching a rotational electrode in close-fittingheat and electrically conductive relation, the electrode being selectedfrom the group consisting of cylindrical and disk electrodes; and e)electrical terminal means for receiving a plurality of externalelectrical connectors that is mounted on the protective member oppositethe portion of the conductive member on which the rotational electrodeis mounted.
 2. The rotational test electrode assembly of claim 1,wherein the mounting means are mating threaded surfaces.
 3. Therotational test electrode assembly of claim 1 which further comprises aplurality of thermocouples in contact with the conductive member, eachof the plurality of thermocouples having an electrical lead in contactwith the terminal means.
 4. The rotational test electrode assembly ofclaim 1, wherein the protective member comprises at least two portions,and at least one of the portions engages a surface of the rotationalelectrode in fluid-tight relation when assembled for operation.
 5. Therotational test electrode assembly of claims 1 which further comprisesmounting means proximate the electrical terminal means for coupling theassembly to rotational drive means.
 6. The rotational test electrodeassembly of claim 1, wherein the heating device is an electricalresistance heater.
 7. The rotational test electrode assembly of claim 1,wherein the conductive member is brass.
 8. The rotational test electrodeassembly of claims 1, wherein the protective member is a formed from anelectrically insulative polymer.
 9. The rotational test electrodeassembly of claim 1, wherein the polymer is selected from the groupconsisting of polytetrafluoroethylene, polyethylene, polypropylene,polyvinyl chloride, and copolymers thereof.
 10. The rotational testelectrode assembly of claim 1, wherein the mounting means for attachingthe rotating disk electrode is a threaded aperture located in the end ofthe solid portion of the conductive member positioned to receive acooperatively threaded shaft extending from the disk electrode, wherebythe surfaces surrounding the aperture and the shaft are in close-fittingrelation when assembled.
 11. The assembly of claim 10, wherein the diskelectrode includes a peripheral shoulder that engages a surface of theprotective member in fluid-tight relation when assembled.
 12. Therotational test electrode assembly of claim 1, wherein the mountingmeans for attaching the cylindrical electrode comprises a threadedsurface formed on the conductive member and extending inwardly from theend defining the solid portion of the conductive member and acooperatively threaded interior surface of the cylindrical electrode,whereby said cylindrical electrode is assembled on the conductive memberto a position proximate the heating device.
 13. The assembly of claim12, wherein the protective member consists of a first cylindricalportion terminating in fluid-tight relation with one end of thecylindrical electrode and second portion that terminates in fluid-tightrelation with the other end of the cylindrical electrode.
 14. Theassembly of claim 13, wherein the second portion is in the form of acylindrical cup having an internally threaded sidewall that iscooperatively received on the threaded surface of the conductive member.15. The rotational test electrode assembly of claim 1, wherein themounting means comprises a reinforcing cylindrical flanged sleeve thatis received in close-fitting relation on the interior of the protectivemember and the flange extends over the end of the protective member. 16.The assembly of claim 15, wherein the flanged sleeve is metal.
 17. Theassembly of claim 15, wherein the end of the sleeve positioned insidethe protective member is displaced from the end of the annular portionof the conductive member by an electrically insulative air gap, wherebythe electrode is electrically isolated from stray electrical currentsduring operation.
 18. The assembly of claim 1 which further comprises anelectrical lead extending from the interior surface of the annularportion of the conductive member to the electrical terminal.